In U.S. patent application Ser. No. 12/928,603, filed on Dec. 15, 2010 and entitled “Method and Apparatus for the Production of High Tenacity Polyolefin Sheet”, there was presented a “direct roll” process for the production of a coherent virtually fully dense polyolefin sheet suitable for further processing by drawing to produce a high tenacity, highly oriented film, tape, fiber, ribbon, or sheet from a polyolefin powder. The direct roll process disclosed therein included feeding a metered amount of polyolefin powder into the nip between two heated calender rolls and rolling the powder through the nip under these conditions until a coherent sheet of polyolefin was produced. The direct roll process eliminated the need for compacting the polyolefin powder with a separate and costly compaction step common to the prior art, which required substantially complex and costly compaction equipment. The preferred polyolefin in the direct roll process was ultra high molecular weight polyethylene (UHMWPE), which formed a non-fibrous, monolithic, high tenacity, ultra high molecular weight polyethylene tape product suitable for use in high strength applications such as ballistic resistant articles or panels. The tape product of the present invention can be slit into narrow widths below ½″ for use in applications such as rope, netting, and sails.
Although the aforementioned patent application adequately provided a high strength tape product for use as the high modulus component in ballistic-resistant products, unexpected beneficial results have recently been obtained with the direct roll process. It has been found that additional improvements in the direct roll process yield UHMWPE tapes with a substantially higher tensile modulus in the range of 1600 to 2500 grams/denier. Such a higher modulus range translates into articles or panels with superior ballistic resistance, or, when slit into narrower widths, into higher modulus tensile members in sail cloth, ropes, slings, or nets. Additionally, processing UHMWPE tapes or sheets into ballistic-resistant products is substantially easier and less costly than processing UHMWPE fibers into ballistic-resistant products. A resin matrix is commonly used to bind multifilament fibers with parallel alignment into uni-directional web products often called uni-tape. The non-fibrous tape of the previous and current invention is already a web with a very high degree of orientation and contains 100% of the critical polymer useful for ballistic protection and none of the binding matrix. A binding matrix can be added between layers and further consolidated into ballistic articles by heat and pressure. Regardless, the total percentage of non-ballistic material in the form of a binding matrix is much smaller for the tape of this invention, offering improved performance.
Current methods of production of high modulus UHMWPE include the gel-spinning technique, a well known method capable of producing multi-filament fibers or bundles of very small monofilaments as described in U.S. Pat. No. 4,551,296. These fibers are produced by various manufacturers including DSM High Performance Fibers in the Netherlands under the trade name Dyneema and by Honeywell in the USA under the trade name Spectra. The molecules of UHMWPE are first dissolved in a suitable solvent to help untangle the molecules and isolate individual chain molecules and the resulting solution, or gel, is spun through a spinneret, the solvent removed, and the resulting fiber, with low-entanglements, is drawn to provide high modulus and tenacity. Because a spinneret is used to produce this material, the denier per filament for the high modulus material after drawing is very low, typically less than 10 denier per filament (dpf) and often less than 1 dpf. The aspect ratio is also small since these fibers are nearly round with an aspect ratio approaching 1:1 in the cross direction. Higher aspect ratios can be produced but the typical aspect ratio is under 10:1. Some differences in manufacturing gel-spun products versus the production of high modulus non-fibrous tape via rolling and drawing according to this invention is a) the use of a solution to dissolve the polymer prior to spinning versus the absence of solvent and no dissolving of polymer in the present invention, b) the formation of small monofilaments with small aspect ratios that are bundled together to form multi-filament fibers compared to the monolithic, homogenous, rectangular shaped tape product produced in this art with an aspect ratio well above 10:1, c) the molecular weight of the gel-spun fibrous product is typically much lower than the molecular weight of the starting gel-spun polymer whereas the tape product of the present invention has the same molecular weight as the starting polymer, d) undesirable small traces of solvent remain in the finished gel-spun product whereas no solvent is used or retained in the tape product of the present invention, e) gel-spun product exhibits high creep values whereas the tape product of the present invention exhibits very low creep, f) gel-spun PE products are known for their high abrasion and cut resistance but the abrasion resistance of the tape product of the present invention exhibits even higher abrasion and cut resistance owing to its monolithic homogenous nature and higher molecular weight, and g) the manufacturing costs and complexity are significantly lower for the tape product of the present invention as the equipment is less capital intensive to buy and install and simpler to operate and there is no use or consumption of solvent during the manufacturing process.